Electrophotosensitive copiers include a photo conductor with a photoconductive layer with a conductive backing. The photoconductor is transported along an endless path relative to a plurality of work stations, each of which is operative when actuated to perform a work operation on the electrophotosensitive medium. Such stations include a charging station at which a uniform charge is placed on the photoconductive layer, an exposure station at which the charged photoconductive layer is image-wise exposed to actinic radiation from the medium to create an electrostatic image of the medium in the photoconductive layer, a developing station at which the electrostatic image is contacted with finely divided charged toner particles for adhering to the photoconductive layer in a configuration defined by the electrostatic image, a transfer station at which such toner particles are transferred in the image configuration to a receiving surface, and a cleaning station at which residual toner is removed from the photoconductive layer so that it can be reused. The electrostatically held toner image is then adhered to the paper by flowing the toner particles together.
The most common method of adhering the toner to the paper is a combination of heat an pressure to fuse the toner to the paper. The fuser typically includes a pressure roller and a fuser roller and a structure for delivering offset preventing oil to the fuser roller. The fuser roller is heated and the pressure roller may or may not be heated to a temperature less than or equal to the fuser roller so that both heat and pressure are applied to toner particles at the nip between the rollers to fix the toner to a receiver sheet in an image-wise configuration. By convention the fuser roller generally described the roller which comes in contact with the unfused toner and is usually the roller having the higher temperature if there is a temperature differential. The term "fuser" is used herein to identify both the fuser and the pressure roller.
The pressure and fuser rollers performance reliability is one of the most important factors which influences the electrophotographic copier life cycle cost and customer satisfaction. These rollers performance are affected by the ability of the not only the fuser roller, pressure roller, and heating rollers, but also the oiler donor roller and metering rollers to remain free of contamination.
One of the long-standing problems with electrostatographic toner fusing mechanism is the adhesion of the heat-softened toner particles to the surface of the fuser rollers, oiler donor roller, and metering roller as well as to the receiver, known as "offset" which occurs when the toner-bearing receiver is passed through a fuser. Toner is passed from the fuser to external heater rollers (if present), pressure roller, and oiler donor roller. Toner is passed from the oiler donor roller to the metering roller and from the metering roller to the metering blade. Toner on the metering roller causes streaks while toner on the metering roller causes errors in the oil laydown. Excessive toner build up on heater rolls and both fuser rollers can lead to image defects, jams, and ultimate roller failure. There have been several approaches to decrease the amount of toner offset onto the fuser roller. One approach has been to make the toner-contacting surface of a roller, for example, a fuser roller and/or pressure roller of a non-adhesive (non-stick) material.
One known non-adhesive coating for fuser roller comprises fluoropolymer resins, but fluoropolymer resins are non-compliant. It is desirable to have compliant fuser rollers to increase the contact area between a fuser roller and the toner-bearing receiver. However, fuser rollers with a single compliant rubber layer absorb release oils and degrade in a short time leading to wrinkling artifacts, non-uniform nip width and toner offset. To make fluoropolymer resin coated fuser rollers with a compliant layer, U.S. Pat. Nos. 3,435,500 and 4,789,565 disclose a fluoropolymer resin layer sintered to a silicone rubber layer which is adhered to a metal core. In U.S. Pat. No. 4,789,565, an aqueous solution of fluoropolymer resin powder is sintered to the silicone rubber layer. In U.S. Pat. No. 3,435,500, a fluoropolymer resin sleeve is sintered to the silicone rubber layer. Sintering of the fluoropolymer resin layer is usually accomplished by heating the coated fuser rollers to temperatures up to 350.degree. C. Such high temperatures can have a detrimental effect on the silicone rubber layer causing the silicone rubber to smoke or depolymerize, which decreases the durability of the silicone rubbers and the adhesion strength between the silicone rubber layer and the fluoropolymer resin layer. Attempts to avoid the detrimental effect of the high sintering temperatures that have on the silicone rubber layer have been made by using dielectric heating of the fluoropolymer resin layer, for example, see U.S. Pat. Nos. 5,011,401 and 5,153,660. Dielectric heating is, however, complicated and expensive and the fluoropolymer resin layer may still delaminate from the silicone rubber layer when the fuser rollers are used in high pressure fusers. In addition, a fuser roller made with a fluoropolymer resin sleeve layer possess poor abrasion resistance and poor heat resistance.
Again, when a fuser roller becomes contaminated with toner, the contamination can be transferred to a heater roller, thus contributing to the failure. The contamination of the fuser roller can also contribute to the paper jam failure in an electrophotographic engine and it is primarily due largely to the inability of the paper itself to release from the fusing roller. The toner release from the rollers in the electrophotographic engine, which is commonly termed as "offset", is sometimes aided by applying a suitable oil on the roller surface. A probable mechanism for the reduction in offset with oil is that oil flowing into the pores on the surface of the roller material forms a barrier. This barrier is an aid in retarding the offset of contamination. The viscosity of various oils and their molecular structure determine whether the oils will be adsorbed to the roller surfaces. The increase in viscosity makes the displacement of oil from the roller surface more difficult. If a high molecular weight oil is adsorbed on the roller surface, entropy considerations suggest that such oil molecules will have to detach from several sites simultaneously, to regain mobility, making detachment less probable.
The surface energy of the roller material, and its surface morphology can also influence the degree of toner release (offset). Other important material properties such as, wear and abrasion characteristics, thermal conductivity and reactivity or bonding with various functional oils, which are normally used to reduce the offset characteristics can contribute to the choice for suitable heater and pressure roller materials. The commonly used roller material such as elastomers and other experimental roller such as hardcoat anodized rollers, teflon, electroless nickel and electroless nickel impregnated with teflon lack one or more shortcomings in material characteristics described above, and contribute to varying degrees of toner offset.
It has been reported that in order to prevent the offset of toner in a fusing system, attempts have been made to provide a toner release agent also sometimes referred to hereinafter as offset preventing oil, in particular, polydimethylsilicone oil (PDMS) which is applied on the fuser roller to a thickness on the order of about 1 .mu.m to act as a toner release material, these materials possess a relatively low surface energy and have been found to prevent toner from offsetting to the fuser roll surface.
According to prior art techniques, the toner release agents may be applied to fuser roll by several delivery mechanisms including wicking (such as Nomex wick), impregnated webs, or by way of a donor roll.
U.S. Pat. No. 4,659,621 to Finn et al wherein a release agent donor roll is described as having a comformable donor surface comprising the crosslinked product of at least one addition curable vinyl terminated or vinyl pendant PDMS, a silane (Si--H) functional PDMS as crosslinking agent, catalyst and finely divided filler. These donor rolls suffer from roller failure because of the tendency to swell while in contact with the silicone release agent which is absorbed into the silicone rubber. The silicone rubber donor roll no longer can provide a uniform layer of release agent to the fuser roller.
U.S. Pat. No. 5,061,965 to Fergusm et al wherein a release agent donor roll has a base member, an intermediate comformable silicone elastomer layer and an elastomer release agent donor layer comprising poly(vinyldene fluoride-hexafluoropropylene-tetrafluoroethylene) where the vinylidene fluoride is present in an amount &lt;40 mole %, a metal oxide present in an amount sufficient interact with polymeric release agent having functional groups to transport a sufficient amount of the polymeric release agent to provide an interfacial barrier layer between the fusing surface and the toner. This donor roller suffers from the oil wetting capability between the nonfunctional PDMS release agent and the nonreactive donor roller surface since the invention counts on the polymeric release agent having functional groups to react with the metal oxide which is dispersed into the fluoroelastomer layer.
U.S. Pat. No. 5,141,788 and U.S. Pat. No. 5,166,031 to Santokh Badesha wherein a release agent donor roll comprising a supporting substrate having an outer layer of a surface grafted or volume grafted polyorganosiloxane formed by dehydrofluorination of said fluoroelastomer by a nucleophilic dehydrofluorinating agent, followed by addition polymerization by the addition of an alkene functionalized polyorganosiloxane and a polymerization initiator. Fabricated donor rolls used for supplying conventional silicone oil release agent showed 4.3 million copies without failure. Although these rolls provide long life, non-oil swelling, and can also be used with non-functional PDMS release agent, the manufacturing of such a donor roll is tedious, inefficient, and expensive.